Technical Field
The present disclosure relates to a reading circuit for a magnetic-field sensor, for example, an anisotropic magnetoresistive (AMR) magnetic sensor.
Description of the Related Art
Magnetic-field sensors, in particular AMR magnetic sensors are used in a plurality of fields and systems, for example in compasses, in systems for detecting ferrous materials, in the detection of currents, and in a wide range of other applications, thanks to their capacity for detecting natural magnetic fields (for example the Earth's magnetic field) and magnetic fields generated by electrical components (such as electrical or electronic devices and lines traversed by electric current).
In a known way, the phenomenon of anisotropic magnetoresistivity occurs within particular ferrous materials, which, when subjected to an external magnetic field, undergo a variation of resistivity as a function of the characteristics of the same external magnetic field. Usually, these materials are applied in the form of thin strips so as to form resistive elements, and the resistive elements thus formed are electrically connected to form a bridge structure (typically a Wheatstone bridge).
It is also known to manufacture AMR magnetic sensors with standard techniques of semiconductor micromachining, as described for example in U.S. Pat. No. 4,847,584. In particular, each magnetoresistive element can be formed by a film of magnetoresistive material, such as for example Permalloy (a ferromagnetic alloy containing iron and nickel), deposited to form a thin strip on a substrate made of semiconductor material, for example silicon.
When an electric current is made to flow through a magnetoresistive element, the angle θ between the direction of magnetization of the magnetoresistive element and the direction of the current flow affect the effective value of resistivity of the magnetoresistive element so that, as the value of the angle θ varies, the value of electrical resistance varies (in detail, this variation follows a law of the cos2θ type). For example, a direction of magnetization parallel to the direction of the current flow results in a maximum value of resistance to the passage of current through the magnetoresistive element, whilst a direction of magnetization orthogonal to the direction of the current flow results in a minimum value of resistance to the passage of current through the magnetoresistive element.
In particular, the Wheatstone-bridge detection structure (which may be defined as “microelectromagnetic”) of an AMR magnetic sensor includes magnetoresistive elements that have ideally the same resistance value and are such as to form diagonal pairs of equal elements, which react in an opposite way with respect to one another to the external magnetic fields, as shown schematically in FIG. 1 (where I is the electric current flowing in the magnetoresistive elements and R the common resistance value).
If a supply voltage Vs is applied at the input of the bridge detection structure (in particular to the first two terminals of the bridge, which operate as input terminals), in the presence of an external magnetic field He, a variation of resistance ΔR of the magnetoresistive elements occurs and a corresponding variation of the voltage drop on the same magnetoresistive elements. In fact, the external magnetic field He determines a variation of the direction of magnetization of the magnetoresistive elements. This results in an unbalancing of the bridge, which causes a voltage variation ΔV at output from the bridge circuit (in particular between the remaining two terminals of the bridge, which operate as output terminals). Since the direction of the initial magnetization of the magnetoresistive elements is known beforehand, as a function of the voltage variation ΔV it is thus possible to determine the component of the external magnetic field acting in the direction of sensitivity of the magnetic sensor (therefore, using three magnetic sensors with directions of sensitivity orthogonal to one another, it is possible to determine the modulus and direction of the external magnetic field).
In particular, in order to detect unbalancing of the Wheatstone bridge and generate an electrical output signal indicating the characteristics of the external magnetic field to be measured, a reading circuit (or front-end) is normally used, which is coupled to the output of the detection structure of the AMR magnetic sensor and includes a signal-conditioning stage, comprising amplification and filtering units. The detection structure and the associated reading circuit together form the magnetic-field sensor, which supplies at output an electrical signal as a function of the detected magnetic field, and has a given input/output response, due in part to the sensitivity of the bridge detection structure, and in part to the gain of the associated reading circuit.
In many applications in which AMR magnetic sensors are normally used, disturbance magnetic fields, usually known as “stray fields” are superimposed on the magnetic signals to be detected; the value of these stray fields may even be comparable to, if not higher than, that of the signals to be detected. The stray magnetic fields, due to the operating environment of the magnetic sensors, act as an offset that is superimposed upon the useful signal to be detected.
For example, AMR magnetic sensors find advantageous application, in particular as compasses, in mobile-phone systems. In these applications, the stray magnetic fields are generated by the same electronic or mobile-phone devices in which the magnetic sensors are incorporated (in particular, with the transceiving antenna operatively active), and may be oriented in the direction of detection of the AMR magnetic sensors. In this case, the value of the useful magnetic field to be detected is, for example, in the region of 0.6 gauss (Earth's magnetic field), whilst the stray magnetic field generated by the electronic or mobile-phone devices can even reach high values, for example 4 gauss.
It follows that a selection in the design stage of the full-scale value of the AMR magnetic sensor, made as a function of the useful signal to be detected, can lead to saturation of the sensor (in particular of the corresponding electronic reading circuit), in the presence of stray magnetic fields having a high value. This saturation in turn entails the impossibility of carrying out a correct reading of the useful magnetic field to be detected.
In particular, the reading circuit typically comprises an analog part for signal conditioning (in terms of amplification and filtering), and possibly a part for analog-to-digital conversion, which supplies the output signals. Usually, it is the output analog-to-digital conversion unit that saturates up to the full-scale value, in the presence of stray components (the analog part may be designed for working in linear dynamics for intervals greater than the maximum dynamics of the analog-to-digital converter). Alternatively, the reading circuit may include just the analog part, and be externally provided with an appropriate analog-to-digital conversion unit. Also in this case, saturations of the analog reading chain may occur in the presence of stray magnetic fields.
To overcome the above drawback, it is possible to use higher full-scale values so as to prevent any saturation during reading, at the expense, however, of obtaining generally lower values of resolution in the detection of the magnetic fields. In fact, in a known way, the full scale of the sensors, in addition to indicating a maximum value of the detectable magnetic field, is in general associated to the measurement resolution, the dynamics of a corresponding output signal being fixed. In other words, an increase in the full scale is to be considered equivalent to a decrease in the resolution of the sensor (for example, in the digital case, this is due to the presence of a higher full scale given the same quantization levels of the analog-to-digital converter). This reduction in the electrical performance of the sensors may, however, not be acceptable, in particular in those applications requiring an accurate measurement of the magnetic field to be detected, such as, for example, in magnetometers.
Moreover, given the unpredictability of the stray magnetic fields that may be superimposed on the useful signal during the effective conditions of use of the associated electronic device (for example, the aforesaid mobile-phone device), it is not in any case convenient to select beforehand, for example in the design stage, an adequate full-scale value for the AMR magnetic sensor, nor ensure, in all the possible conditions of use, the absence of saturation of the corresponding electronic reading circuit.
AMR magnetic sensors that have been so far proposed are hence not altogether satisfactory and optimized as regards the selection of the full-scale value, and frequently are unable to ensure the desired results in terms of the corresponding electrical performance.